The invention relates to a molding machine apparatus and method for the production of thermoplastic skins or shells by means of what is generally known as rotational or slush molding. One example of a prior art apparatus is described in my issued U.S. Pat. No. 6,461,561.
A continuing operational challenge and technical objective for this type of molding is to quickly heat the mold to a uniform temperature. This is because the surface area of the tooling is irregular. It is an object of this invention to heat the mold tooling uniformly, rapidly and efficiently to thereby minimize cycle time for production of parts.
According to the present invention, a plurality of high velocity streams of heated air are directed at the irregularly shaped tooling in order to heat the mold to the operating temperature. In particular, an array of combustion conduits are arranged around the tooling which each direct the heated air at different parts of the tooling to thereby bring the tooling to operating temperature.
It has been found that heat is most efficiently transferred to the tooling when the air flow over the surface thereof is turbulent. Turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities. When flow is turbulent, air exhibits additional transverse motion which enhances the rate of energy and momentum exchange thus increasing the heat transfer and the friction coefficient.
As the velocity of the air over the surface of the tooling increases beyond the point of establishing turbulent flow, the amount of heat or energy transferred does not increase proportionally. Because increased turbulent flow does not proportionately heat the tooling surface to a higher temperature, all surfaces of the tooling experiencing turbulent flow will tend to a uniform temperature.
Because the tooling has an irregular shaped surface, some surface areas of the tooling will experience turbulent flow before other surfaces and these surfaces will heat more rapidly than those not subject to turbulent flow. It is accordingly an object of the invention to be able to control, adjust and vary the molding machine operating parameters to achieve turbulent air flow over the entire surface of the irregular shaped tooling. As such, the molding machine elements and operating parameters should preferably be adjustable. In order to make good parts, the operator must be able to control and change these parameters as required.